This invention relates to a method of signal injection, transmission, interconnection (termination), and detection, and to a power transmission network, ie. a mains electricity distribution and/or transmission network, and a filter therefor. In particular it relates to the use of mains electricity networks and/or lines for telecommunications transmission (e.g. voice, data, image and/or video).
In the UK, it is conventional to describe a power network for 33 kV and above as a "transmission network", and one for less than 33 kV as a "distribution network". In this specification the term "electricity distribution and/or power transmission network" is normally used, but general references to power networks and to transmission of signals are to be construed as applying to all such networks.
Traditionally telecommunications signals have been transmitted on independent networks e.g. telephone lines. More recently, in order to simplify and increase efficiency of telecommunications services to domestic or industrial premises, there have been investigations into using existing electricity transmission and distribution networks to carry telecommunications services.
A transmission network and a filter therefor is described in the applicant's co-pending International Patent Application, PCT/GB93/02163. This describes the use of telecommunications signals having a carrier frequency greater than around 1 MHz on a power transmission network. The teaching and disclosure of that patent application should be referred to in relation to the present invention, and is incorporated herein by reference.
At such carrier frequencies, it has been found that the cables of a power transmission and/or distribution network exhibit pseudo-coaxial characteristics and therefore attenuation of a signal transmitted along the cables is reduced. In this way, both speech and data signals can be transmitted at carrier frequencies of greater than approximately 1 MHz, allowing for a larger available spectrum and greater transmission capacity.
However, it has been found that the attenuation effects still limit the distance over which signals of a given carrier frequency and bandwidth can effectively be transmitted. On, for example, a 415 volt network the carrier frequency may preferably be between 1-10 MHZ, and on, e.g., an 11 kV network maybe between e.g. 1-20 MHZ or possibly 5-60 MHZ. Signals of this frequency can be transmitted over large distances (using repeater stations if necessary) and therefore a network used in this way is suitable for general telecommunications and telephony signals.
However broadband telecommunications signals, such as television communications, generally require a higher carrier frequency (or frequencies) and bandwidth. As indicated above, such signals are therefore usually transmitted on an independent broadband telecommunications network e.g. co-axial cable, optical fibre etc.
A carrier frequency of up to hundreds of MHZ may be used on a network as described in PCT/GB93/02163, although the distance over which a signal with a particularly high carrier frequency may be transmitted is limited by the dynamic range sensitivities and power levels deployed on the network--possibly to around 7-40 metres on a typical UK network.